This invention relates to an improvement on a field controlled thyristor and a method for producing the same. One type of field controlled thyristor which is well known to those skilled in the art is a current controlling device in which an anode region and a cathode region are formed in and exposed to the two opposed main surfaces of a semiconductor substrate having one conductivity type, and a buried gate region is provided in the substrate to surround a current path from the anode to the cathode region. When such a field controlled thyristor is supplied with a reverse bias voltage, with its gate region and cathode region kept negative and positive, respectively, a space charge or depletion region extends around the gate region into the current path region so that the current path from the anode to the cathode region is electrically interrupted by the depletion region to cut off the anode-to-cathode current. For embedding the gate region, epitaxial growth after the formation of the gate region has been employed.
U.S. Pat. No. 4,060,821 issued to Houston et al, on Nov. 29, 1977, discloses a field controlled thyristor wherein the semiconductor substrate has two opposed main surfaces and a cathode region of N.sup.+ -type conductivity with its surface exposed in one of the main surfaces of the substrate, a gate contact region of P.sup.+ -type conductivity, a burried gate or grid region of P-type conductivity, a current path or channel region of N.sup.- -type conductivity and an anode region of P.sup.+ -type conductivity with its surface exposed in the other main surface of the substrate are repeatedly formed in the lateral direction in the semiconductor substrate, with the adjacent buried grid regions of P-type conductivity spaced from each other by a predetermined distance.
The buried grid region is in contact with the gate contact region which is a surface-exposed region. A cathode electrode and a gate electrode are attached respectively on the cathode and the gate contact regions while an anode electrode is provided on the anode region. When a suitable reverse bias voltage is applied between the cathode and the gate electrodes, a space charge region expands through and occupies the spacing between the adjacent buried grid regions to interrupt the current flowing from the anode to the cathode. In this field controlled thyristor disclosed in the above U.S. Patent, the forward blocking voltage gain is increased by decreasing the spacing between the adjacent buried grid regions.
In the above field controlled thyristor, the forward voltage drop can be reduced by making the impurity concentration in a first portion of the channel region intervening between the cathode and the buried grid region higher than that in a second portion of the channel region intervening between the buried grid and the anode region, as has been suggested by Terasawa et al in the U.S. Pat. No. 4,223,328 filed on June 1, 1978 and issued on Sept. 16, 1980. The high impurity concentration in the first intervening portion of the channel region will improve the injection efficiency of the carriers being injected from the cathode region into the first and the second intervening portion of the channel region, thereby reducing the forward voltage drop.
On the other hand, since in this field controlled thyristor the P-N junction between the first intervening portion and the buried grid region is inversely biased, the P-N junction must usually have an inverse blocking voltage of higher than 50 V. Although the forward voltage drop decreases with the increase in the impurity concentration in the first intervening portion, the inverse blocking voltage of the P-N junction between the first intervening portion and the buried grid region is also lowered with the increase in the impurity concentration of the first intervening portion. In the field controlled thyristor disclosed in the above U.S. Pat. No. 4,223,328, there is thus a limitation, in view of the inverse blocking voltage, for increasing the impurity concentration in the first intervening portion of the channel region to provide a sufficiently low forward voltage drop. Also, in the field controlled thyristor described in the U.S. Pat. No. 4,223,328, the buried grid region lies between the first and the second intervening portions of the channel region which is of higher resistivity than the buried grid region, and is in contact with the gate contact region of P.sup.+ -type exposed in the first main surface of the semiconductor substrate. These gate contact and buried grid regions are formed through the combined use of diffusion process and epitaxial growth. As is well known, the epitaxial growth technique is a more complicated and delicate process than the diffusion process, thereby resulting in poor production yield and, therefore, high production cost. Accordingly, such a field controlled thyristor having a buried gate and produced by the combination of diffusion and epitaxial growth is necessarily an expensive one.